1. Field of the Invention
The invention relates to pliant, flexible and breathable polyolefin microporous films formed from at least one layer, which are permeable to gases and water vapor but impermeable to aqueous liquids, have high mechanical properties and have a pleasant feel.
The invention also relates to a process for producing, by flow extrusion, polyolefin microporous films consisting of at least one layer, which films are permeable to gases and water vapor but impermeable to aqueous liquids, have high mechanical properties and have a pleasant feel.
Finally, the invention relates to the use of the aforementioned films by themselves or combined in the form of a complex with other porous supports.
2. Description of Related Art
It has been known for a long time how to produce pliant and flexible microporous films having the ability to breathe, that is to say to be permeable to gases and to water vapor but impermeable to aqueous liquids, and to combine them with other materials such as macro perforated films, nonwoven fabrics, woven fabrics and papers, these other materials providing the mechanical properties and the desired feel, particularly the textile feel.
The most conventional method for producing these microporous films consists in preparing a compound formed from a matrix of thermoplastic polymers having elastic properties and from a pulverulent, generally inorganic, filler, in flow-extruding this compound in the melt into the form of a thick web, in taking this thick web up by means of a metal roll (the surface of which may be provided with patterns in order to emboss the web) having a function of prestretching the molten web and cooling it, then in stretching it at least uniaxially, after having reheated it to a temperature favorable for this operation, in order to obtain the desired film.
During the stretching (post-cooling) allowing the thick web to pass into the state of a thin film, the inorganic filler particles dispersed within the thermoplastic polymer matrix are at least partially separated from the polymer material by a mechanical effect, creating microchannels within and passing through the thickness of the film.
The choice of the hydrophobic character of the polymer matrix and that of the inorganic filler, and in particular the size of its particles, the quality of the dispersion of the filler within the polymer matrix and the choice of stretch ratio applied to the web in order to form the film, are as many factors which favor the creation of these microchannels (the equivalent diameter of which is a few microns) and which give the film its porosity with respect to gases and its impermeability to aqueous liquids.
The prior art shows that microporous breathable films have formed the subject of considerable research relating to:
the composition of the polymer matrix and the mineral fillers, such as calcium carbonate and barium sulfate (U.S. Pat. No. 4,472,328) and the addition of elastomers (such as polybutadiene) in order to improve the stretchability;
the process for manufacturing the film, and in particular an operation of embossing the flow-extruded and slightly stretched web, in order to create variations in film thickness (2 xcexcm to 3 mm) therein over very short distances, and also the process of combining it with other materials in order to increase the mechanical strength and/or the quality of the feel.
By way of illustration, mention may firstly be made of European Patent EP 232 060 which describes a process for producing a gas-permeable porous film according to the following steps, which comprises:
forming a compound from a polyolefin resin and an inorganic filler;
flow extruding a ribbon from the molten compound;
prestretching, embossing and cooling the molten ribbon on a metal roll in order to give it a rough configuration;
and finally carrying out the stretching proper, uniaxially or biaxially, after reheating it, so as to form the microporous film and impart a macromolecular and crystalline orientation sufficient to give it all the desired properties.
According to that process:
the polyolefin resin may comprise homopolymers, such as polypropylene, low-density polyethylene, linear low-density polyethylene, high-density polyethylene and polybutylene, copolymers, such as ethylene-propylene copolymer and ethylene-vinyl acetate copolymer, or blends of these polymers. Particularly preferred are polypropylene, low-density polyethylene, linear low-density polyethylene or high-density polyethylene, or a blend of two of more of these polymers;
and the inorganic filler is preferably barium sulfate or calcium carbonate.
Within this same context, another process may also be cited, this having formed the subject of European Patent No. EP 283 200. This other process appears to be very similar to that described in the above patent since it is carried out according to the same steps but which, however, is distinguished therefrom:
by the composition of the film formed, which is a blend of a single ethylene/C4 to C10 alpha-olefin copolymer and of an inorganic filler (calcium carbonate);
and by the pattern of the embossing which appears to be a selection of a geometrical shape (hexagonal, circular or diamond shape).
In either of these processes, the molten compound forming the composition of the film is flow-extruded in the form of a hot thick web. This web is almost simultaneously prestretched, cooled and embossed by means of a metal cooling and embossing roll which is provided with a surface pattern, bringing the thickness of the completely cooled web to about 100-150 xcexcm.
Since the embossed web coming from the embossing operation does not yet have the desired characteristics for the microporous film, such as, in particular, a small thickness and a microporosity giving it the ability to breathe while being impermeable to aqueous liquids, or mechanical properties necessary for its subsequent use, it undergoes, after having been reheated to the suitable temperature (for example by calendering), a stretching operation between two pairs of rollers (forming a stretching rig) which rotate at different speeds.
The first pair of rollers constitutes the forwarding pair which simultaneously fulfils the roles of delivering the embossed ribbon to be stretched and possibly of being an aid for maintaining the delivered web at the desired temperature for stretching.
The second pair of rollers constitutes the pair for the actual stretching of the reheated embossed ribbon, by applying the desired stretch ratio to it, this ratio being given by the ratio of the linear speed delivered by the pair of stretching rollers to that delivered by the pair of forwarding rollers.
After the stretching operation, the film made according to the process may undergo a heat treatment, generally under tension in order to control its shrinkage.
Thus, it is obvious that the prestretched molten web entering the embossing zone is consequently and almost fully cooled therein and that this embossed web has to be reheated to a temperature sufficient to be able to be correctly stretched in the appropriate stretching zone and converted as far as possible into a film having the expected properties.
However, when the web coming from the flow extrusion is prestretched, cooled and embossed, the constituent material of the web, which is initially in an almost isotropic state, undergoes internal physical modifications during these steps, particularly macromolecular deformations and/or a macromolecular orientation and/or change in the crystallinity, these being frozen-in during the cooling.
Although the web coming from the embossing/cooling is reheated in order to undergo in the best possible state the actual stretching step (between the two pairs of rollers), this reheating appears always to be insufficient to completely release the material of the web from its internal state frozen by the cooling. This is why the final stretching of the embossed web takes place on an already stressed material, which adds further stresses to those existing at the time of stretching. This accumulation of internal stresses is manifested by the amount of shrinkage by heating the film leaving the stretching unit, which often requires post-stretching thermal stabilization in order to at least partially remove these stresses.
Consequently, the film coming from the prestretching, cooling, embossing and then stretching steps according to the prior art cannot be in a sufficiently isotropic physical state as is desirable even if it has, by virtue of these steps, many desirable characteristics.
This why:
a first object of the invention is to provide an improved process for manufacturing a microporous film which is permeable to gases and to water vapor but impermeable to aqueous liquids, is at least a monolayer and is polyolefin-based, another object of the invention is to provide a process for manufacturing a microporous film, which is at least a monolayer, having a physical state with a high isotropic tendency after stretching the cooled web, that is to say an essentially amorphous and homogeneous physical state;
another object of the invention is to have a microporous film, which is at least a monolayer, having satisfactory permeability to gases and impermeability to aqueous liquids, formed by at least uniaxially stretching at least one web prepared from a compound based on at least one polyolefin polymer and at least one mineral and/or organic filler.
Consequently, the invention relates both to an improved process for producing an at least monolayer microporous film permeable to gases but impermeable to aqueous liquids, and to the film itself.
According to the invention, the process for producing a microporous film having a high isotropic tendency, permeable to gases but impermeable to aqueous liquids, being composed of at least one layer, comprising the following successive steps:
preparing a compound composed of a polyolefin matrix comprising at least one polymer and at least one mineral and/or organic filler;
extruding a web comprising at least one layer by melt flow of the compound;
prestretching the web by a drive roll;
cooling and solidifying the prestretched web, by means of the drive roll;
stretching the web, at the temperature suitable for forming the microporous film, is characterized in that:
the cooling of the prestretched molten web by the drive roll is partial and limited in a controlled manner to a temperature lying within the range of temperatures necessary for stretching it;
the web taken to the necessary temperature by the partial cooling is stretched by pulling it at the moment when its tangential separation from said drive roll takes place, said roll acting as a forwarding roller for the stretching operation.
According to the invention, the microporous film, permeable to gases but impermeable to aqueous media, is characterized by an isotropic, homogeneous and partly amorphous physical state.